Comet's minerals hint at liquid water

The comet crashed into by NASA's Deep Impact spacecraft on 4 July 2005 contains material apparently formed by liquid water and not ice, according to new observations with the Spitzer Space Telescope.

This could suggests the disc of gas and dust from which the solar system formed got mixed together billions of years ago, whisking matter from warm regions near the Sun outward - or that an unknown process may allow a layer of liquid water to exist beneath the dusty coatings on comets.

Spitzer was one of the 80 or so telescopes trained on Comet 9P/Tempel 1 when it rammed into the 370-kilogram copper-tipped impactor sent into its path. A spectrometer on the telescope detected a mix of materials as they streamed off the comet, including crystallised silicates, clay and carbonates.

Clay and carbonates are thought to form in liquid water, which can only exist for long periods on the surface of planets or other objects no further from the Sun than Mars. But comets such as Tempel 1 are thought to have formed as smaller chunks of material smashed together around the orbit of Neptune - where any water would be in the form of ice.

"How did clay and carbonates form in frozen comets?" says Carey Lisse, an astronomer at the University of Maryland in College Park, US, who observed the impact with Spitzer. "We don't know, but their presence may imply that the primordial solar system was thoroughly mixed together, allowing material formed near the Sun where water is liquid, and frozen material from out by Uranus and Neptune, to be included in the same body."

Dredged up

This argues against the standard theory that dust and gas in a disc around the Sun simply clumped together with other material at the same distance from the star.

The mixing up of solar system material could happen if the disc behaved like molasses, with clumps of material pulling in their surroundings as they swirled in spiral paths around the newborn Sun, about 4.5 billion years ago. "You can dredge material from near the infant Sun all the way out," says Lisse.

But he acknowledges he is "going out on a limb" with his interpretation and that another process might allow the carbonates to form inside the comet at its present distance.

"I'm thinking more that there may have been a [suitable] environment in the comet itself," says Paul Weissman, a comet researcher at NASA's Jet Propulsion Laboratory in Pasadena, California, US.

Rewrite the books

He says the comet is heated by the Sun, and this heat can travel downward some distance into the comet. "It doesn't seem to get that far down because we see ice excavated from the impact," he told New Scientist.

But if it can travel through the dust that coats comets - which may be up to a metre thick, it may melt some of the water ice. It is not clear whether the carbonate-forming reaction would need a process that prevents the added pressure - created by any melting - from venting water into space.

The discovery of carbonates "creates problems one way or another," he says. But if the mixing theory is true, "it would rewrite how we think about the solar nebula".

The research was presented on Wednesday at a meeting of the American Astronomical Society's Division of Planetary Sciences in Cambridge, UK.

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